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Planet-Gobbling Star

Posted by timothy on from the this-is-my-theory dept.

crymeph0 writes "BBC is carrying a story about a star that mysteriously brightened three times last year. Scientists now know why. It's been eating gas-giant planets that orbit it! I'm just glad Earth isn't a tasty gas planet, or else we'd have to start making sacrifices to Sol to play it safe." It's hard to prove things from 20,000 light years away, but this explanation is interesting.

14 of 85 comments (clear)

  1. Right then... by Anonymous Coward · · Score: 5, Funny

    so it's just a case if indigestion, then?

  2. I read a paper on something related to this by Zachary+Kessin · · Score: 4, Interesting

    In stars where there is a much higher level of "metals" seem to generate more large planets than we have. It seems that in a star like this you may end up with 4-10 Jovian size planets. In the case of our solar system you have 2 very large planets and everything is far enough appart that it is stable. On the other hand if you had a bunch of planets at 10 Jovian masses it is inevitable that a few would be kicked out of the system and a few put into very close in orbits.

    it all comes down to how much matter there is to create planets. The higher the densisty of heavy elements the faster things start to clump into planets, and the bigger the planets get.

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  3. uh-oh by Tumbleweed · · Score: 3, Funny

    Somebody call Commodore Decker and Captain Kirk!

  4. a new standard candle? by Ckwop · · Score: 3, Interesting

    Perhaps we have a new standard candle in the making here. Perhaps this effect is closely tied to the starting mass and composition of the solar system of the star.. and thus the brightness is roughly the same for each event..

    Just a thought :)

    Simon

  5. In related news.. by ewhenn · · Score: 4, Funny

    Prevacid stock has spiked sharply up.

  6. Sci Fi nuttery from me by Henry+V+.009 · · Score: 3, Interesting

    I imagined for a second an advanced civilization crashing gas giants into their sun to keep it alive a little longer. I am wrong, of course, and this is no doubt the work of gravity, but I would like to point out that if we ever decide that we would like to keep our Sun burning for an extra million years or so, the only way to do that will be to crash Jupiter into her. On the other hand, the energy expended to do that would probably be better expended in creating environs that can support life without a sun.

  7. Time for a Mars bar, yum! by G4from128k · · Score: 4, Funny

    OK, its more a Jupiter bar -- a chewy metallic hydrogen center covered in rich fluffy methane-ammonia clouds. What every growing star needs for a burst of energy.

    How many orbits does it take to get to the center of a gas-giant lollipop?

    --
    Two wrongs don't make a right, but three lefts do.
  8. A Couple Thoughts/questions by CheshireCatCO · · Score: 3, Interesting

    1. They're positing that eating one or three giant planets is enough new fuel to make the star brighten significantly? (I wish the article had the details on how much it actually brightened.) A typical gas giant is around 1/1000 the mass of the parent star. That's not a lot of new fuel, particularly when you consider that the star has way more hydrogen than that left over from main sequence burning.

    2. My most recent understanding (and I admit that I'm only half paying attention to this) is that the planets-contaminate-stars model for the heavy element enrichment probably doesn't explain the observed enrichment. (Probably because the planet's bits would have to stay right near the star's surface over the long run. See mass ratio, above.)

    I'm not saying that this model doesn't work, but I'm skeptical. I'd really want to see their stellar models showing how addition of a giant planet's mass of hydrogen on the surface of the red giant affects the luminosity. I'd also like to see evidence that this star had planets before the brightening. (I wouldn't be shocked if the data didn't exist. But I still want to see it. :-)

    1. Re:A Couple Thoughts/questions by CheshireCatCO · · Score: 3, Informative

      1) See, I asked about how much it *brightened*. Not how bright it *got*. I noted that line, too., but knowing that it is now 600,000 L_sun isn't really helpful in telling us how much brighter it is now than before. We would need to also know what its starting point was. This makes a difference: if the star brightened by 0.1%, the possible mechanisms are quite different from the star brightening by a factor of 100.

      2) Um, no. When a star gets to Fe (and only very large stars do), it makes a nice little explosion adn we enrich the interstellar medium. Which is where pretty much all of the "metals" (anything heavier than helium, according to astrophysicsists) in your body, Earth, the Sun, etc. come from. So the question about metal-rich stars isn't "are they producing the metals", they would have had to leave the main sequence for one thing. The question is did the cloud that formed them have an more metals than the average, or did the metals get preferentially introduced by, say, planets smacking in to them.

      No, see, as star is WAY bigger than a planet. (By definition, almost.) So a planet, particular a gas giant which is in large part hydrogen and helium (10s of percent and up, by mass) smack into the star, unless the material stays right near the surface, all of those metals will basically be so thinly spread throughout the volume of the star that you'll never see a real enrichment to within error bars.

      And remember, the volume of a shell goes like the radius or the star squared, so the thickness of the shell has to be pretty thin to keep an appreciable fraction of the metals. Say we want to spread the metals out over a volume roughly equal to the volume of the original core. Uranus is mostly core, so let us use its radius as the radius of the core. (Note: much of Uranus's core is hygrogren compounds, as are all giant planet cores. This means that we're *over*estimating the volume of metals.) And lets spread it over a spherical shell on the Sun's surface.
      V_Uranus = 4/3 pi r_u^3
      V_shell = 4 pi r_s^2 deltaR
      where r_u is Uranus's radius (2.62E9 cm) and r_s is the Sun's (6.9E10 cm), deltaR is the thickness of the spherical shell, and the Vs are volumes. Equating and cancelling, we get that deltaR = r_u^3/3 r_s^2. Plugging in numbers, that's a thickness of about 1.3E6 cm, or about 0.0018 % of the Sun's diameter. Which, when you consider that the Sun is fluid and convection does happen (although the most convective part is a bit lower down below the surface), isn't a whole lot. Confining the metals to that region would be very difficult.

      This would probably be why current thinking tends more towards the "the clouds that formed star with planets were unusally rich in metals." Also, it makes sense: more metals, more stuff to actually *build* planets with.

    2. Re:A Couple Thoughts/questions by barawn · · Score: 3, Informative

      No - you don't fuse iron. You neutron-stuff them. During the supernova, the outbound shock wave carries so much energy (and neutrinos) that neutrons are literally "shoved" into nuclei. You get ridiculous things like iron with hundreds of neutrons, which then decay down into normal elements by alpha emission and beta decay. This is r-process stellar nucleosynthesis. (There's also s-process stellar nucleosynthesis, which is also neutron stuffing, but on a much longer timescale. Essentially everything past iron is formed by r-process stellar nucleosynthesis. Check Carroll & Ostlie pp 527-528 for more info.

      Stars die when they hit the iron stage because they can generate no outward pressure from fusing iron. They can't even fuse iron at all! It's actually a really complex procedure - basically, the iron starts to lose all of its electrons (from proton capture and other mechanisms) so the core rapidly loses electron degeneracy pressure, which is what was (briefly) supporting it. The inner core collapses very uniformly to a little neutron star, and the outer core decouples from the inner core, and the outer core rushes inwards at extreme velocities. The collision of the two is one of many explosions in a supernova. (Again, see Carroll & Ostlie's section on the Death of Massive Stars)

      Anyway, the fuel isn't insignificant depending on what stage the star is in, and also depending on how fast the planet's orbit would decay once it's inside the photosphere. If it meets with the star's core without significantly losing mass, that could cause a VERY large brightening. Functionally it's equivalent to a nova, or the pulsing of Wolf-Rayet stars (without the mass shell shielding it).

    3. Re:A Couple Thoughts/questions by CheshireCatCO · · Score: 3, Informative

      Depends on the kind of star. The core of the star - that is, the "dense" part -

      In this case, it's an F-class star. And you missed my point entirely, which was that if the star can convect the planet's hydrogen into the shell-burning zone, it can damn well convect its own hydrogren reserves down there, which are vastly in excess of what the planet could provide. So the star should never notice the miniscule addition of the planet's hydrogen.

      This is all theory, of course, but unfortunately, theory doesn't quite bear out the "hydrogen compounds = gas giant planet cores".

      See, that's where you're amazingly wrong. Let's review out giant planets, shall we? (If you want, I suggest you crack open Protostars and Planets IV; it's always good to actually do a bit of research.)

      Jupiter May or may not have a core in the first place. If it does, it's at most around 10 Earth-masses (maybe as high as 15, but that's at the outer edge of the error bars). Mostly, it'll be hydrogen compounds with some rock and metal (real metals, not in the astrophysical sense). You need that core in standard formation models before you can accrete the hydrogen and helium gas. The metallic hydrogen is a layer right above the core, not the core itself.

      Saturn Has a core, around 10-15 Earth-masses. Same as Jupiter in composition. This is easier to work out in theory because the equation of state is better understood for the interior pressures within Saturn. (Jupiter's higher pressures make things more dicey.)

      Uranus and Neptune Definately have cores. Also icey with a bit of metals and rock thrown in. Again, need said core to hold on to the gas in the first place. Cores are pretty well constrained in size at around 15 Earth-masses in both planets. Given that both planets are around 18 Earth-masses in size, you bet your ass that this means that they are both mostly core. In fact, it's this that has lead some leading researchers to dub them "ice giants", in contrast to Jupiter and Saturn, the "gas giants."

      I don't know where you got your "facts", but they're pretty much uniformly wrong. See Wuchterl et al. in P&P IV for more details on constraints on the present structures of these planets.

  9. hmmm by GypC · · Score: 3, Informative

    If it's 20,000 LY away it didn't brighten 3 times last year (that we know of)... rather it brightened 3 times in one year approximately 20,000 years ago.

  10. This has been written before by devphil · · Score: 3, Informative


    Read Niven's A World Out Of Time (multiple meanings in the title) for a similar idea. It's one if his first "State" books.

    SPOLIERS BELOW

    Basically, something else gets dropped into Jupiter. And there's some fascinating ideas on how to move a planet around.

    --
    You cannot apply a technological solution to a sociological problem. (Edwards' Law)
  11. Unlikely? by Alsee · · Score: 3, Interesting

    It's a facinating theory, but there's a huge problem with it. Planetary orbits are highly unstable unless they are pretty widely spaced. It is therefore pretty much impossible that there were THREE planets anywhere near the same distance away from the sun.

    Seeing it happen to three different stars in one year, OK. Seeing it happen three times to one star over thousands or millions of years, OK. But there's no was a single star ate 3 planets in a single year without some HUGE outside influence disrupting the orbits.

    If the theory is right then it is of secondary interest, and whatever triggered the triple event is probably far more important and interesting.

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